JSB's seem to be the best in mine but it's only running at about 8ft/lbs. As you know they have a sloping breech so any pellet that goes right into the breech & does not leave the tail sticking out will be better than a tight fitting one. If it's still got the leather breech seal it will benefit from having an O ring fitted. I just bedded mine in with soft compound glue from a glue gun. took a few attempts to get it right but works a treat when you do . Out of curiosity if you chrono it could you let me know what it's doing?